I'm wondering if that's not Foul (650 potency)

If we look at it with the raw potency itself, it's Midare Setsugekka indeed (720 potency).
However, to calculate the strongest hit, we have to consider passive traits and self buff.

Samurai has a 10% self damage buff to setup on himself and another 10% damage debuff on the target.
Black Mage has a passive trait that increase every action's damage by 30% and Enochian which increase damage by 20%.